Electric machines are well known in the art. They can be either of the electric motor type that produces a force when electric energy is supplied, or of the generator type that generates electric energy when a mechanical force is applied to the generator. There exists both rotary and linear electric machines. A problem with electric motors is that the torque the motor can deliver is limited by the ferro-magnetic core which will saturate at a certain current level. Above this current level, the torque will not increase but the heat produced in the core will increase. When the core is heated, the magnetic properties will deteriorate as is the case with most magnetic materials.
The problem is particular troublesome when it comes to linear motors. The moving part of the motor, in this application referred to as the rotor, is often limited in size and thus the maximum electric field that can be applied to it is limited. This leads to an insufficient acceleration in different applications.
Different possible solutions to solve the problem with the limited delivered torque are known in the art. For a stationary motor, this can be dealt with, for example, by increasing the size of the motor. In applications where size and weight are of importance, different exotic materials are used in the ferro-magnetic core. The core can also be laminated in different ways, and different geometric layouts are known in which the core is “distributed” in the magnetic field in an optimum manner.
In applications where a linear motor is used to control an electrically actuated valve, the limited torque is apparent. To be able to provide a somewhat useable solution, the present known systems for controlling an electrically actuated valve utilize a system comprising an electromagnet and a spring set up in a resonant system. In this way it is possible to adjust the valves somewhat. These systems are difficult to control and it is only possible to adjust the valves slightly during a cycle.
In applications where an electric machine is used in connection with a free piston engine, a conventional linear electric machine can be used to take out the average power when the piston is in a resonance state.
It is desirable to control the movement of the piston in each cycle, for instance, to control the combustion. This is difficult to achieve with conventional electric machines since such machines have a relation between the electric force and the mass of the rotor which is about ten times too low.